Is there an app whose only purpose would be to change the wifi prefs, by starting or stopping the wifi on iPod Touch? I would like to have an icon on the home page, and a simple click on it will switch the state of the wifi.

As an alternative, if you're running iOS 4+ you can go to the Settings icon and then there's the 'Flight mode' option at the top which does just that. Not as a simple as you were hoping for, but it is just a click and a swipe.

I have a Linksys WAG160N which my iPod disagrees with so I frequently have to reset my wifi this way, and it's much quicker than the old iOS 3 workflow.